Calorie restriction is known to prolong average and maximum life spans in many species. One of the most widely accepted theories of aging is the oxidative stress theory which hypothesizes that oxidative damage produced by reactive oxygen species (ROS) accumulates over time leading to the development of diseases of aging such as cancer, cardiovascular disease, frailty and eventually death. Since ROS is a byproduct of energy metabolism, we hypothesize that calorie restriction will cause a metabolic adaptation (decrease in energy expenditure larger than accounted for by the decrease in metabolic body size) associated with a decrease in ROS-mediated oxidative damage. More specifically, we hypothesize that the decrease in energy expenditure (24-hour sedentary and sleeping) measured in a respiratory chamber will be larger than that expected on the basis of the changes in fat-free mass and fat mass and that this adaptation is associated with reduced activities of the sympathetic and thyroid systems. Furthermore, we will determine whether the magnitude of this metabolic adaptation is related to the calorie restriction-induced decrease in markers of oxidative stress to lipids (urinary isoprostanes), proteins (in blood) and DNA (nucleated blood cells). We will determine for the first time whether the metabolic adaptation is long lasting and dependent on the state of energy balance since studies will be performed after 1 year (probably still in negative energy balance) and after 2 years (now in energy balance) of 25% calorie restriction. The proposed study will be conducted as an ancillary study of a randomized clinical trial of calorie restriction (CALERIE). Importantly, our data will be compared to those obtained on mitochondrial function measured in vitro and in vivo in another ancillary study (PI, SR Smith). PUBLIC HEALTH RELEVANCE: The proposed studies have been designed to test one of the major hypotheses explaining why caloric restriction increases lifespan in almost all animal species. This hypothesis rate of living theory states that life expectancy of an animal is inversely related to its metabolic rate per unit of weight. We propose to test whether a lowering of the rate of living (energy metabolism) is associated with an accompanying decrease in body temperature and oxidative stress in 50 volunteers consuming 25% less calories per day for 2 years. Both low body temperature and low oxidative stress are predictive of longer life.